The present invention generally relates to semiconductor devices and more particularly to a quantum semiconductor device including quantum dots and a fabrication process thereof.
In a so-called bulk crystal where there is no confinement of carriers, it is well known that the density of state of the carriers increases continuously and parabolically with energy. In a quantum well structure in which carriers are confined one-dimensionally in a crystal, there appear discrete quantum levels as is well known in the art. In such a case, the density of state of the carriers changes stepwise. Because of such a restriction imposed on the distribution of the carriers, a quantum well structure provides a narrow spectrum when used for an optical semiconductor device such as a laser diode, and the efficiency of laser oscillation is improved. Further, a quantum well structure is used in electron devices having a resonant tunneling barrier such as an RHET (Resonant Hot Electron Transistor) as an energy filter of carriers.
In a quantum well wire structure in which the degree of confinement of the carriers is increased further, the density of state of the carriers in the crystal is modified such that the density of state is a maximum at the bottom edge of each step. Thereby, the sharpness of the spectrum is increased further.
In an ultimate quantum dot structure in which the degree of carrier confinement is increased further, the density of state becomes discrete in correspondence to the discrete quantum levels. A system having such a discrete energy spectrum, in which transition of carriers occurs only discontinuously or stepwise, provides a very sharp spectrum when used for an optical semiconductor device even in a room temperature environment where the carriers experience substantial thermal excitation.
Further, the quantum dot structure is drawing the attention of scientists in relation to the problems of fundamental physics such as a phonon bottleneck problem of energy relaxation.
Conventionally, a quantum well structure has been formed readily and with reliability by using an MBE (Molecular Beam Epitaxy) process or an MOCVD (Metal Organic Chemical Vapor Deposition) process such that a very thin quantum well layer is sandwiched between a pair of barrier layers. On the other hand, a quantum well wire has been formed by growing thin semiconductor layers laterally on a so-called inclined semiconductor substrate having a stepped surface structure. Alternately, a quantum well wire may be formed by applying an electron beam lithography to an ordinary, one-dimensional quantum well structure.
Thus, various attempts have been made to form quantum dots by using an inclined substrate similarly to the case of forming a quantum well wire. However, such conventional attempts have faced a problem of controlling the stepped surface of the inclined substrate. Further, there tends to occur a mixing of elements at the boundary of the quantum dots formed such a manner. Thereby, a desired sharp transition of the composition is difficult in the quantum dots formed as such. Thus, there are few successful attempts in the approach that use an inclined substrate in combination with lateral epitaxial growth of semiconductor layers. In addition, an approach to use electron-beam lithography to form a quantum dot is also unsuccessful due to the damage caused in the epitaxial layers forming the quantum dot or a barrier layer at the time of patterning.
On the other hand, there has been a discovery that a quantum dot can be formed easily by using a so-called S-K (Stranski-Krastanow) mode growth that occurs in a strained heteroepitaxial system such as an InAs/GaAs heteroepitaxial structure at the initial period of heteroepitaxial growth. In an S-K mode growth, quantum dots are formed in the form of discrete islands on a substrate. For example, it is reported that an MBE growth of an InGaAs layer having an In-content of 0.5 on a GaAs substrate with a thickness of several molecular layers, results in a formation of islands of InGaAs each having a diameter of 30-40 nm on the GaAs substrate (Leonard, D., et al., Appl. Phys. Lett. 63, pp.3203-3205, 1993). Further, it is reported that islands of InGaAs having a diameter of 15-20 nm are formed on a GaAs substrate by an ALE (Atomic Layer Epitaxy) process with a mutual distance of about 100 nm (Mukai, K., et al., Jpn. J. Appl. Phys., 33, pp.L1710-L1712, 1994). Further, a similar quantum dot can be formed also by a MOVPE process (Oshinowo, J. et al., Appl. Phys. Lett. 65, (11), pp.1421-1423, 1994).
As the formation of a quantum dot in such a strained heteroepitaxial system is controlled by a strain energy formed at the heteroepitaxial interface, the formation of the quantum dot is substantially simplified as compared with the conventional process discussed previously. Further, the formation of a quantum dot on a strained heteroepitaxial system does not require a patterning process and is inherently free from damages. There is already a report claiming successful observation of a photoluminescence (PL) (Leonard, D., et al., op. cit.), in which it is reported that a broad PL peak is confirmed in the vicinity of 1.2 eV with a substantial intensity.
In the quantum dots formed by the S-K mode growth, however, the observed PL peak, although having a substantial intensity, spreads or diffuses substantially. For example, the half-height width FWHM (Full Width at Half Maximum) of the PL peak spreads over a range of 80-100 meV, probably due to the poor control of the size of the individual quantum dots. It should be noted that the PL wavelength corresponding to the foregoing PL peak energy is about 1.1 xcexcm, which is shifted substantially on a shorter wavelength side with respect to the wavelength of 1.3 xcexcm that is used commonly in the field of optical telecommunication and optical information processing. With the conventional S-K mode quantum dots, it has been difficult to tune the PL wavelength as necessary. As will be described later, the size of the quantum dot itself can be controlled to some degree by controlling the deposition temperature. However, the size of the quantum dots formed in such a manner changes variously. It is believed that it is such a variation of the size of the quantum dots that causes the foregoing unwanted spreading of the PL peak. Further, it is believed that conventional quantum dot structure thus formed by the S-K mode growth includes a substantial number of quantum dots that do not contribute to the photon emission.
Recently, Farad et al., (Farad. S., Appl. Phys. Lett., 68 (7), pp. 991-993, Feb. 12, 1996) has reported a successful observation of a PL wavelength in the 1.5 xcexcm-band for an S-K mode quantum dot of InAs formed on an AlInAs buffer layer, which in turn is provided on an InP substrate with a lattice matching therewith. In this case, however, the value of FWHM for the observed PL spectrum exceeds 110 meV, indicating that there still remains a substantial problem in the size control of the individual quantum dots.
Thus, conventional S-K mode quantum dots have failed to provide a sharp spectrum of photon emission in the wavelength band of 1.3 xcexcm or 1.5 xcexcm, which is important for industrial applications. Further, a similar problem occurs also in electron devices such as an RHET. An energy filter formed by conventional S-K mode quantum dots performs poorly when the S-K mode quantum dots are used in a resonant-tunneling barrier of an RHET. In such a case, the desired sharp tunneling effect is not obtained.
Accordingly, it is a general object of the present invention to provide a novel and useful quantum semiconductor device including quantum dots and a fabrication process thereof wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a quantum semiconductor device including a plurality of quantum dots formed on a heteroepitaxial system according to an S-K growth mode, wherein the quantum levels can be set independently to the size of the individual quantum dots, as well as a fabrication process of such a quantum semiconductor device.
Another object of the present invention is to provide a quantum semiconductor device including a plurality of quantum dots coupled with each other quantum-mechanically such that the plurality of mutually coupled quantum dots form collectively a plurality of discrete quantum levels.
Another object of the present invention is to provide a quantum semiconductor device, comprising:
a semiconductor substrate;
an active layer formed on said semiconductor substrate and including a quantum structure,
said quantum structure comprising:
a plurality of intermediate layers stacked on each other repeatedly, each of said plurality of intermediate layers being formed of a first semiconductor crystal having a first lattice constant;
each of said intermediate layers including a plurality of quantum dots of a second semiconductor crystal having a second lattice constant different from said first lattice constant, said second semiconductor crystal forming thereby a strained system with respect to said first semiconductor crystal, each of said quantum dots in an intermediate layer having a height substantially identical with a thickness of said intermediate layer;
a quantum dot in an intermediate layer aligning with another quantum dot in an adjacent intermediate layer in a direction perpendicular to a principal surface of said semiconductor substrate;
each of said plurality of intermediate layers having a thickness equal to or smaller than a Bohr-radius of carriers in said intermediate layer.
Another object of the present invention is to provide an optical semiconductor device, comprising:
a semiconductor substrate;
a first cladding layer provided on said semiconductor substrate;
an active layer provided on said first cladding layer and including a quantum structure;
a second cladding layer provided on said active layer;
a first electrode provided on said semiconductor substrate for injecting carriers of a first type into said active layer via said first cladding layer;
a second electrode provided on said second cladding layer for injecting carriers of a second type into said active layer via said second cladding layer;
said quantum structure comprising:
a plurality of intermediate layers stacked on each other repeatedly, each of said plurality of intermediate layers being formed of a first semiconductor crystal having a first lattice constant;
each of said intermediate layers including a plurality of quantum dots of a second semiconductor crystal having a second lattice constant different from said first lattice constant, said second semiconductor crystal forming thereby a strained system with respect to said first semiconductor crystal, each of said quantum dots in an intermediate layer having a height substantially identical with a thickness of said intermediate layer;
a quantum dot in an intermediate layer aligning with another quantum dot in an adjacent intermediate layer in a direction perpendicular to a principal surface of said semiconductor substrate;
each of said plurality of intermediate layers having a thickness equal to or smaller than a Bohr-radius of carriers in said intermediate layer.
Another object of the present invention is to provide a method of fabricating a quantum semiconductor device including a quantum dot structure, comprising the steps of:
(a) growing quantum dots epitaxially on a substrate in the form of mutually isolated islands, each of said quantum dots forming a strained system with respect to said semiconductor substrate;
(b) growing a semiconductor layer epitaxially on said semiconductor substrate so as to bury said quantum dots to form an intermediate layer, said semiconductor layer forming said intermediate layer forming a lattice matching with respect to said semiconductor substrate;
(c) growing quantum dots epitaxially on said intermediate layer in the form of mutually isolated islands, each of said quantum dots grown on said intermediate layer forming a strained system with respect to said semiconductor substrate;
(d) growing a semiconductor layer epitaxially on said intermediate layer formed in said step (b) so as to bury said quantum dots formed in said step (c) as a next intermediate layer, said semiconductor layer forming said next intermediate layer forming a lattice matching with respect to said semiconductor substrate;
said steps (c) and (d) being carried out alternately and repeatedly,
said intermediate layer being formed, in each of said steps (b) and (d) such that said intermediate layer has a thickness smaller than a height of said quantum dots in a state that said quantum dots are grown freely.
According to the present invention, the S-K mode quantum dots are aligned in a direction perpendicular to the principal surface of the semiconductor substrate and collectively form a single effective quantum dot isolated from other similar effective quantum dots. In other words, each of the effective quantum dots formed in such a manner is isolated and forms quantum levels similar to a real quantum dot. By controlling the number of stacks of the S-K mode quantum dots, it is possible to control the quantum levels formed in such a manner as desired. By forming the intermediate layer to have a thickness smaller than the height of the S-K mode quantum dots, mechanical as well as quantum mechanical coupling of the vertically aligned S-K mode quantum dots is enhanced.
Another object of the present invention is to provide a resonant-tunneling diode, comprising:
a semiconductor substrate;
a first quantum structure formed on said semiconductor substrate;
a barrier layer formed on said first quantum structure, said barrier layer forming a potential barrier; and
a second quantum structure formed on said semiconductor substrate;
a cap layer provided on said second quantum structure;
a first electrode provided in electric contact with said semiconductor substrate; and
a second electrode provided in electric contact with said cap layer;
each of said first and second quantum structures comprising:
a plurality of intermediate layers stacked on each other repeatedly, each of said plurality of intermediate layers being formed of a first semiconductor crystal having a first lattice constant;
each of said intermediate layers including a plurality of quantum dots of a second semiconductor crystal having a second lattice constant different from said first lattice constant, said second semiconductor crystal forming thereby a strained system with respect to said first semiconductor crystal, each of said quantum dots in an intermediate layer having a height substantially identical with a thickness of said intermediate layer;
a quantum dot in an intermediate layer aligning with another quantum dot in an adjacent intermediate layer in a direction perpendicular to a principal surface of said semiconductor substrate;
each of said plurality of intermediate layers having a thickness equal to or smaller than a Bohr-radius of carriers in said intermediate layer;
said quantum dots aligning in each of said first and second quantum structures forming collectively a single effective quantum dot having quantum levels.
According to the present invention, it is possible to eliminate the leakage current substantially completely at the tuned-off state of the quantum-resonant tunneling diode.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.